The invention relates generally to television measuring apparatuses and more particularly it relates to an apparatus for automatic measurement of the video signal-to-noise ratio in a television channel.
There are apparatuses known in the art for automatic measurement of the video signal-to-noise ratio in a television channel, their operation being based on amplitude modulation of short pulses by noise contained in the video signal and on subsequent processing of modulated short pulses to determine the r.m.s. value of the noise.
The prior art apparatus for automatic measurement of the video-signal-to-noise ratio in a television channel comprises a unit measuring the peak-to-peak amplitude of the video signal, one input of which is fed with the video signal and which is controlled by a control unit and a serially connected sync separator, one output of which is connected with another input of the unit measuring the peak-to-peak amplitude of the video signal and to the input of which is applied a video signal, a short pulse shaper and an pulse amplitude modulator to modulate short pulses with the video signal, which is coupled to a pulse stretcher the output of which is electrically coupled to the input of a function generator, the output of the latter being coupled through an analog-to-digital converter to the input of a digital display, the other input of which is connected to the output of the control unit, the pulse stretcher being electrically coupled to the function generator by means of a filter.
The disadvantage of the existing apparatus for automatic measurement of the video signal-to-noise ratio in a television channel is that a filter is connected to the input of the function generator to convert the stretched short pulses into a continuous signal, which after square-law detection, integration and logarithmic conversion carried out by the analog method in the function generator is reconverted to a pulse form in the analog-to-digital converter. Therefore the noise being measured is subjected to a triple conversion a continuous form at the input of the apparatus -- a discrete form at the output of the pulse amplitude modulator -- a continuous form at the filter output -- a discrete form at the output of the analog-to-digital converter. Such triple conversion plus the analog function conversion increases the conversion error and consequently, considerably reduces the accuracy of noise measurement.
Another disadvantage of the prior art apparatus is that in the noise measurement cycle it fails to adequately cancel the video signal and suppress various types of interference, including low-frequency power supply hum, line frequency interference and its harmonies and spurious signals of the pick-up tubes. This considerably reduces the resulting measurement accuracy, especially when the noise level is measured in the picture signal.
Still another disadvantage of the prior art apparatus for automatic measurement of the video signal-to-noise ratio in a television channel is that the video signal containing the noise being measured is applied to the input of the pulse amplitude modulator either directly or through a weighing filter which attenuates the noise. This results in the low sensitivity of the entire apparatus and thereby limits the measurement range towards the large video signal-to-noise ratios, i.e. low noise levels.
The prior art apparatus is also disadvantageous in that the noise measurement performed by this apparatus is not free from the effect of sharp variations of the video signal levels which occur when the picture brightness or the scene is abruptly changed. This considerably decreases the accuracy with which the noise level is measured in the video signal corresponding to a moving picture.